A typical prior art head and disk system 10 is illustrated in block form in FIG. 1. In operation the magnetic transducer 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer 20, usually called a “head” or “slider,” is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded.
The conventional disk 16 for longitudinal recording includes substrate 26 of glass or AlMg with an electroless coating of Ni3P that has been highly polished. The thin films 21 on the disk 16 typically include one or more chromium or chromium alloy underlayers 33 (see FIG. 2) and at least one ferromagnetic layer 34 based on various alloys of cobalt. For example, a commonly used alloy is CoPtCr. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer 35 is used to improve wearability and corrosion resistance. Various seed layers 32, preseed layers 31 and multiple underlayers 33 have all been described in the prior art.
The layer structure shown in FIG. 2 can be used with a variety of magnetic layer stacks 34. For example, two or more laminated magnetic layers can be used and antiferromagnetically coupled layers structures can be substituted for any or all of the magnetic layers.
A simple version of a laminated magnetic layer stack 34 (FIG. 3) includes an upper magnetic layer 36 and a lower magnetic layer 38 that are separated by nonmagnetic spacer layer 37. It is known that substantially improved signal-to-noise ratios (SNR) can be achieved by the use of a laminated magnetic layer structure. The reduced media noise is due to the effective doubling of the media grains by means of suppressing the exchange coupling between the magnetic layers which allows the upper recording layer to switch independently from the lower magnetic layer. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer thicknesses from a few angstroms upward that result in the best decoupling of the magnetic layers and the lowest media noise.
FIG. 4 shows an embodiment of a prior art laminated magnetic media in which the lower magnetic layer is replaced with an antiferromagnetically coupled (AFC) structure 41 comprising an AFC-master magnetic layer 42, an AFC spacer 43 and an AFC slave magnetic layer 44.
In U.S. Pat. No. 6,280,813 to Carey, et al. a layer structure is described that includes at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic coupling/spacer film. In general, it is said that the exchange coupling oscillates from ferromagnetic to antiferromagnetic with increasing coupling/spacer film thickness and that the preferred 6 Angstrom thickness of the ruthenium coupling/spacer layer was selected because it corresponds to the first antiferromagnetic coupling peak in the oscillation for the particular thin film structure. Materials that are appropriate for use as the nonferromagnetic coupling/spacer films include ruthenium (Ru), chromium (Cr), rhodium (Rh), iridium (Ir), copper (Cu), and their alloys. Because the magnetic moments of the two antiferromagnetically coupled films are oriented antiparallel in Carey's media, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically coupled films add constructively. An embodiment of the structure includes two ferromagnetic CoPtCrB films, separated by a Ru spacer film having a thickness selected to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. The top ferromagnetic layer is designed to have a greater Mrt than the bottom ferromagnetic layer, so that the net moment in zero applied magnetic field is low, but nonzero. The Carey '813 patent also states that the antiferromagnetic coupling is enhanced by a thin (5 angstroms) ferromagnetic cobalt interface layer added between the coupling/spacer layer and the top and/or bottom ferromagnetic layers. The patent mentions, but does not elaborate on the use CoCr interface layers.
Published European patent application EP1059629 by Abarra, et al. describes the use of a nonmagnetic Ru-M3 coupling layer 221 with a thickness from 4 to 10 angstroms to couple the adjacent magnetic layer in an antiparallel orientation, i.e., antiferromagnetic coupling. The application lists the M3 elements as Co, Cr, Fe, Mn, Ni and alloys thereof. The nonmagnetic coupling layer of Abarra is said to have a thickness range of approximately 0.4 to 1.0 nm in order to establish the antiparallel magnetizations of the magnetic layers. (See paragraph 0141). In order to maintain the hcp structure of the coupling layer, the embodiment using Ru—Co is limited to 0 to 50 at % Co. (See paragraph 0144).
One disadvantage that laminated media has is that it generally needs to be thick so that each of the magnetic layer is independently stable. The large Mrt increases the pulse width at 50% amplitude (PW50), which reduces media resolution and makes ultra-high linear recording densities more difficult to achieve. The overall thickness of the media can also be a problem for the writing process with the lower magnetic layer 38 being rather far away from the write head.
The previous discussion relates to longitudinal recording. In perpendicular magnetic recording the orientation of the magnetic domains is perpendicular to the plane of the recording layer on the disk. A common type of perpendicular magnetic recording system uses a single write pole type of recording head and a recording medium includes a ferromagnetic recording layer (RL) over a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The perpendicular recording layer is selected to have its c-axis oriented substantially out-of-plane or perpendicular to the plane of the recording layer. Granular ferromagnetic cobalt alloys, such as a CoPtCr alloy, can be designed and fabricated for use in perpendicular recording. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity media and to reduce intergranular exchange coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL can be achieved by the addition of oxides, including oxides of Si, Ta, Ti, Nb, Cr, V, and B. These oxides tend to precipitate to the grain boundaries and together with the elements of the cobalt alloy form nonmagnetic intergranular material.